Characteristics And Genetic Mechanism Of The Dolomite Reservoir Of Qiulitage Formation In Tazhong And Tabei Areas,Tarim Basin

Deep dolomite reservoir has become one of the major targets for further hydrocarbon exploration in Tarim basin.It has been proved by the drilling data that the widely distributed dolomite in Qiulitage Formation of Upper Cambrian is a favorable deep reservoir and should be paid attention to.As a hot research area of the carbonate reservoir in Tarim basin,its main type,characteristics,genetic mechanism and development model should be further studied.According to core observation and thin section identification,combined with cathodoluminescence imaging and geochemical testing results,types with no research value were eliminated.Based on genesis and structural feature,three types of dolomite of Lower Paleozoic are recognized,including the syngenetic-penecontemporaneous stage dolomite,non-hydrothermal origin buried crystal dolomite and structurally controlled hydrothermal origin crystal dolomite.Dolomite reservoir of Qiulitage formation has undergone multiphase superimpose reconstruction by diagenesis like the dolomitization,corrosion and cementation,thus various types of reservoir space are developed like the intercrystalline pore,dissolution vugs and compound types.Because of the excessive regrowth of dolomite crystals,strata with only intercrystalline pores can't form a large-scale favorable reservoir.The late stage karstification is the key factor of forming favorable reservoirs.The study has proved that the favorable reservoir of Qiulitage formation in Tabei and Tazhong is mainly karst reservoir,hydrothermal reservoir and karst-hydrothermal superimposed reservoir.Thus the development mode of favorable reservoir is established.Karst reservoir is mainly developed near the Tabei uplift Tg8 interface,especially in the reef flat facies at platform margin.A great number of dissolution vugs and even caves can be found developed along the strata in the cores.Under microscopic observation,most of the dissolution vugs are in the shape of gulf,with little recrystallization around and filled with no hydrothermal mineral.This kind of reservoir is controlled by facies,the type and distribution of unconformity.The main impact on hydrothermal reservoir is deep thermal fluid,it's common in Tazhong area,the development of reservoir is closely related to fault system,unconformity surface fluid pathway and so on,fluorite,sphalerite,authigenic quartz and some epithermal mineral association include the saddle dolomite are always be found in dissolution vugs or caves.The thermal fluid will change the geo-chemical features and metasomatic original rock of surrounding rock and lead to hydrothermal recrystallization and geopetal structure.The surrounding rock with high porosity could also promote the formation of reservoir with the high quality,so the hydrothermal karst reservoir is controlled by faults system,unconformity surface and sedimentary facies.The dolomite of Qiuliutage Formation in Tabei area takes a long time to form karstified dolomite reservoir by weathering and leaching,at the same time,we can find a lot of evidences of hydrothermal activities.In the dissolution vugs,the mutual intercalary growth of idiomorphic quartz and dolomite and the cathodoluminescence near the quartz indicate that the late stage hydrothermal effects superimposed on the karst reservoirs and formed the karst-hydrothermalt superimposed reservoir.